1. Field of the Invention
The invention relates to a device for manufacturing and preferably also for filling and sealing thermoplastic sacks. This device comprises a forward draw unit for intermittently pulling forward a thermoplastic blown film web and for its hanging feed in a vertical direction through a cross welding and cross severing mechanism, which provides the leading end of the blown film web with a cross weld and severs from said web a tubular segment above a gripping and transport mechanism.
2. Description of the Related Art
This class of device is known from the prior art in different designs. They serve the purpose of providing tubular segments with bottom welds. Preceding this step, the tubular segments were severed from a blown film web, which was wound on a supply roller. Then the welded tubular segments are filled with loose material; and the filled sacks are sealed with head welds and transported away. It is the desire of manufacturers to make such machines work more efficiently, i.e. at a high cycle rate. The output of such machines is also limited due to the fact that the leading end of the plastic web, provided with a bottom weld, must be passed in such a manner through the welding jaws, executing the bottom weld, and the cross cutter that the web is stretched. Furthermore, the freshly formed bottom weld must cool down in order to solidify before the segment, which was severed from the blown film web and now forms the a sack, can be filled with loose material. Pushing the leading end of the blown film web, provided with the bottom weld, in the direction of the transport plane of the sacks to be filled causes the problem that the freely advanced end segment tries to turn aside due to its limpness. Hence, it can move crossways and escape so that it takes additional time until the tubular segment to be severed has assumed its correct vertically hanging position.
Therefore, the object of the invention is to provide a device of the class described above, in which the leading end of the blown film web can be pushed quickly and without limply withdrawing to one side through the cross weld and cross severing mechanism into a stretched hanging position, in which the end sided bottom weld is in the area of the transport plane of the sacks to be filled.
The invention solves this problem in that there is a wall parallel to the feed path of the blown film web, and above the wall or in its top region there are blowing air nozzles, facing the feed direction, between the wall and the feed path.
The blowing air, blown through the blowing air nozzles into the gap between the web and the advanced end of the blown film web, flows in essence laminarly in the feed direction and takes with it the advancing end of the blown film web so that said web is stretched. In this manner a vacuum, which prevents the undesired flapping and turning aside of the fed in end of the blown film web, is generated between the blown film web and the wall. Preferably the blowing air nozzles comprise a row of blowing air nozzles or a elongated nozzle slit.
The blowing air, which serves to feed the end of the blown film web so as to be stretched, also cools the bottom weld. However, to reduce the cycle period for producing and filling the sacks even more, it is desirable to cool the bottom weld so that it will solidify even faster. Therefore, another embodiment of the invention provides that the end region of the feed path has jaws that cool the bottom weld. These jaws can comprise a stationary jaw and a jaw that interacts with the stationary jaw and can be swung like pliers.
Preferably at least one of the jaws is provided with blowing air nozzles aimed at the bottom weld. Preferably both jaws exhibits rows of blowing air nozzles that are aimed at the bottom weld. The jaws clamp the bottom weld preferably not between their jaws in order to prevent the jaws from adhering to said weld.
In devices of the kind disclosed in the invention, sacks of different sizes and length are usually produced and filled. Therefore, it is necessary to change the distance between the cross weld and cross cutting mechanism and the transport plane of the sacks to be filled and to adjust the distance to the varying lengths of the sacks. Therefore, the jaws that serve the cooling process are mounted expediently on the support of a conveyor belt that carries away the filled sacks and that is mounted and guided in the machine frame so that it can be raised and lowered in accordance with the change in size.
The pliers holding the severed sacks can be moved preferably transversely or swung relative to the machine frame so that they can convey the severed sacks by one station length in the direction of the fill station.
Preferably there is a second pair of cooling jaws, of which at least one jaw is cooled and which are spaced one station away from the cooling jaws. Preferably this second pair of cooling jaws is also fastened to the support of the conveyor belt or to a bracket connected to said support.
An especially preferred embodiment of the invention provides that the length of the wall can be adjusted to the height of the support of the conveyor belt, thus to the changed sack size.
To make the adjustment to the different sack sizes, the wall can be made of a flexible web, whose bottom end is fastened to the support or to a bracket connected to said support. In this respect the web is looped between an upper deflecting rod or deflecting edge and an attachment of its other end to the machine frame; and a weight, e.g. a rod, is inserted into the loop. When the support of the conveyor belt moves, the wall adjusts itself automatically to the different sizes in that the wall elongating web segments are pulled out of the loop or wall shortening web segments are pulled into the loop.
To make the adjustment to the different sizes, the wall can also comprise overlapping wall sections, of which the top section is fastened rigidly to the frame and the bottom section is fastened to the support. In this embodiment the wall sections are made of a rigid material.